Cellular polymeric masses are important components of many commercial products, as they are adaptable to a wide variety of functions including, without limitation, insulating, floatation, sound-deadening, cushioning, filtering, packaging, reinforcing and the like. Such masses are commonly referred to as foams. They contain cells, pores, voids or other spaces which are referred to herein as cells. By techniques already well within the skill of the art, the foams may be formed with varying densities and cell sizes and with varying degrees of intercommunication between cells, depending on their intended use. Thus, predominantly closed-cell foams are used for floatation, while open-cell foams are used for filtration.
Thermal stability is the ability to withstand prolonged exposure to elevated temperature without undue loss of strength, distortion, melting, or decomposition. Without thermal stability, polymeric foams are virtually useless for certain structural and thermal insulating applications.
As the present invention employs reaction products of phenolic resins and polyisocyanates, it is of interest to note that these reactants have previously been combined in the attempt to make adhesives, coatings and foams. U.S. Pat. No. 3,242,107, reports the known fact that phenolic resins prepared under acid conditions can be reacted with isocyanates. However, the resulting urethanes revert into the phenol and the isocyanate at temperatures above about 150.degree.C and therefore have very poor heat stability. The urethanes in question of course include bonds formed by reaction of "nuclear" or "phenolic" hydroxyl groups in the resin structure with isocyanate groups in the polyisocyanate to form the urethane bond. The instability of such bonds is widely known. In fact, this instability is used to advantage to "block" isocyanate compounds; that is, in order to temporarily deactivate them, the isocyanates are often reacted with phenolic compounds to form temporary urethane bonds from which the isocyanate may be released and returned to its original unreacted state by gentle to moderate heating. Thus, as reported in U.S. Pat. Nos. 3,268,467 and 3,226,276, such bonds are both temporary and deficient in bond strength. As reported in the literature, Industrial and Engineering Chemistry, December 1962, "The Thermal Dissociation of Blocked Isocyanates", unblocking or the decomposition of the product of reaction of an isocyanate and a phenolic hydroxyl group may commence at temperatures as low as 50.degree. to 80.degree.C.
In view of the foregoing, one would expect that products derived from reacting phenolic resin with isocyanate would generally have poor heat stability. In recognition of this, the art has turned: to the use of phenolic resins from phenols with alkyl substituents located in the para position; to the replacement of the phenolic hydroxyl groups of the phenolic resin with hydroxyalkyl groups, which are known to form more stable bonds with isocyanates, U.S. Pat. Nos. 3,127,373, 3,144,419 and 3,265,641; and to simultaneous employment of both of these measures, U.S. Pat. No. 3,242,107, discussed above. The present invention is believed to be non-obvious, in that it makes possible the production of heat stable foams without the necessity of employing the above-mentioned measures.